"The pilots have never made a 7500 - the international code for 'my plane is hijacked'; it is the pure lack of transponder information (that is suspicious).

"For myself, you would look at the communications panel just behind the throttle quadrant on the Boeing 777 to either switch it off there, or even just removing the fuses."

Mark Weiss, a former commercial pilot now working as an aviation consultant, said he agreed that someone in the cockpit had probably disabled the transponder.

"I don't believe it was an explosive decompression," said Mr Weiss.

"In the US a few years ago we had a Southwest Airlines plane that had an explosive decompression and still maintained the basic structural integrity of the aircraft."

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Gallery: Malaysia Mystery: Search For Missing Plane

Malaysian Air Force close the door of a search and rescue mission flight in Kuala Lumpur, Malaysia.

Disabling of the transponder might lend weight to the theory that one of the pilots sabotaged the flight.

However, it is also possible it was turned off for other reasons - for example if it was giving out faulty data.

Malaysia Airlines has opened an investigation into the conduct of one of the pilots, Fariq Abdul Hamid, after pictures emerged of himself with two women he allowed into the cockpit during a flight two years ago.

However, it has said it has "no reason to believe" the pilots had anything to do with the disappearance.

At a news conference on Thursday, Malaysia's transport minister, Hishammudin Hussein, denied reports the homes of crew members had been searched by police.

Video:Airline Boss Criticises Jet Rumours

The plane was last heard from when the pilot responded to a message confirming it was moving from Malaysian to Vietnamese air traffic control.

The pilot replied "Okay, received, goodnight", but Vietnamese officials have said they never heard from flight MH370.

Not all aviation experts agree that foul play is likely to be to blame for the disappearance.

Dr Colin Brown, director of engineering at Institution of Mechanical Engineers, told Sky he still believes the plane was ripped apart by "explosive decompression".

However, he admitted that the lack of any wreckage or 'black box' signal was hard to make sense of.

Video:Malaysia Defends Missing Jet Search

"This thing is 300 tons of metal with 239 people on board - I cannot understand how you can hide something that size."